Question

tính nhanh

\(A=\frac{5}{20.22}+\frac{5}{22.24}+....+\frac{5}{79.90}\)

\(B=\frac{1}{1.2.3}+\frac{1}{2.3.4}+....+\frac{1}{18.19.20}\)

làm nhanh cho mk nha!!! bn nào trả lời đúng mk cho 2 tick

Answer 1

A = 5/20.22 + 5/22.24+...+5/79.81

A = 5/2 . (2/20.22 + 2/22.24 + ... + 2/79.81)

A = 5/2 . (1/20 - 1/22 + 1/22 - 1/24 + ... + 1/79 - 1/81)

A = 5/2 . (1/20 - 1/81)

A = 5/2 . 61/1620

A = 61/648

Answer 2

B = 1/1.2.3 + 1/2.3.4 + ... + 1/18.19.29

2B = 2/1.2.3 + 2/2.3.4 + ... + 2/18.19.20

\(\Rightarrow\)B = 1/1.2 + 1/2.3 + ... + 1/19.20

\(\Rightarrow\)B = 1/1.2 - 1/19.20

B = 1/2 - 1/380

B = 189/380